A porous honeycomb structure constituted by cell partition walls (ribs) forming an assemblage of a plurality of cells adjacent to each other and a honeycomb outer wall surrounding and holding the outermost cells constituting the circumference of the assemblage of cells, are in wide use as a filter (a diesel particulate filter, i.e. a DPF) for trapping and removing the particulate substance contained in a particle-containing fluid such as diesel engine exhaust gas or as a catalyst carrier for carrying a catalyst component for purification of harmful substance in exhaust gas. Also, refractory silicon carbide (SiC) is in use as a material for the above porous honeycomb structure.
Development works are being pushed forward for a DPF (a DPF for catalyst regeneration) which comprises a conventional DPF and an oxidation catalyst carried thereon and which can oxidize and burn the particulates deposited on the catalyst to continuously regenerate the catalyst.
As such a honeycomb structure, there is disclosed, for example, a porous silicon carbide-based catalyst carrier of honeycomb type which is obtained by forming a silicon carbide powder (a raw material) having a desired specific surface area and containing impurities, into a formed material of intended shape, drying the formed material, and firing the dried formed material in a temperature range of 1,600 to 2,200° C. (e.g. Patent Literature 1).
In the case of the catalyst carrier disclosed in Patent Literature 1, in the sintering (necking) by the recrystallization reaction of the silicon carbide powder per se, the silicon carbide component vaporizes from the surface of the silicon carbide particles and condensates at the contact areas (necks) between the silicon carbide particles, whereby the necks grow and a bonded state results. However, the vaporization of silicon carbide requires a very high sintering temperature, which has invited a high cost; moreover, a material of high thermal expansion coefficient need be sintered at a high temperature, which has resulted in a low sintering yield. Further, when it is attempted to produce a filter of high porosity, particularly high porosity of 50% or more by the above-mentioned sintering by the recrystallization reaction of the silicon carbide powder per se, the sintering mechanism does not function sufficiently and the growth of necks is hindered and, as a result, the filter obtained has had a low strength.
Incidentally, in DPF, it is important to reduce, as much as possible, the pressure loss which has a large influence on the output of an engine. To achieve this task, it is required to allow the DPF to have a higher porosity, that is, to use, as the porous material constituting the DPF, a material of higher porosity. Also, for DPF catalyst regeneration, it is required to suppress the pressure loss as much as possible, by using higher porosity, specifically, a porosity of 50% or more, particularly about 70%.
For obtaining a honeycomb structure having a higher porosity, there is a conventional method of adding a pore former such as starch, foamed resin or the like to a mixed raw material for a porous material (containing silicon carbide particles, etc.) constituting the honeycomb structure to be produced and, during firing of a formed material obtained, burning and blowing off the pore former. The amount of the pore former added need be large in order to achieve a porosity of certain level or higher, for example, a porosity of 60% or more; however, addition of a pore former of organic compound type in a large amount results in generation of a large amount of a gas (e.g. a volatile organic substance and carbon dioxide) during degreasing (calcination), as well as in generation of large combustion heat. The calcinated material (material after degreasing) or fired material obtained under such conditions has, in some cases, defects such as cracks, tears, cuts, large pores caused by agglomeration of organic pore former, and the like, that is, defects which do not allow proper functioning of filter and cause leakage of fluid.
Patent Literature 1: JP-A-6-182228
The present invention has been made in view of the above-mentioned problems of prior art and aims at providing a silicon carbide-based porous material which is high in porosity and strength and superior in oxidation resistance and thermal shock resistance and, when used as a filter, is very low in risk of having defects such as cuts (they cause leakage of fluid) and the like as well as in pressure loss; a honeycomb structure constituted thereby; and a method for producing a silicon carbide-based porous material having the above-mentioned properties.